If you're proactive and try to keep your batteries charged, you're in good shape. However, problems arise when some are charged and ready to go and others are not. You can tell by putting it in your device, turning it on, and checking, but you can save some time by putting a "ready flag" on the battery to denote its charged and ready for use.

Photographer Damon Hair came up with this neat tip, and it applies to more than just charged batteries. When you've finished wiping a flash card is another example. If you have items that are low on batteries, such as a Bluetooth keyboard, you could put a red flag on it to remind you to change the batteries soon.